This invention relates to aircraft having single surface membraneous airfoils, or wings, and a means for increasing the lift obtainable therefrom.
Single surface flexible lifting wings have many applications due to low cost and simplicity of construction. One such application is explained in my U.S. Pat. No. 4,667,898 entitled "Aircraft With Single Surface Membraneous Airfoils", issued May 26, 1987. In that patent, a single surface membraneous airfoil, deployable from a stowed position in the fuselage of an aircraft to a fully extended position by pivotal spars attached to the leading edges, is disclosed. Each membrane airfoil is made from an inextensible, nonporous membrane such as two thousandths (0.002) inch thick stainless steel fixedly fastened to a rigid spar to produce a parabolic or triangular planform. It therefore has been demonstrated that a single-surface membraneous airfoil is suitable for in-flight control of an aircraft. Further, that such membraneous airfoils are aerodynamically stable and are suitable for selected types of vehicles, such as remotely-piloted vehicles (RPV).
Aerodynamic research involving wings and airfoils continues to search for ways to improve the lifting characteristics of airfoils by whatever means for purposes of improved flight parameters, larger payload capability or better handling characteristics at predetermined velocities. Towards this end, U.S. Pat. No. 4,867,396 to Barnaby S. Wainfan, issued on Sep. 19, 1989, and assigned to the Lockheep Corporation discloses that it is known to fixedly attach a small flap-like device to the trailing edge of an aircraft airfoil to improve the coefficient of lift and reduce the coefficient of drag thereof. This flap is a flat, plate-like member having a length of between 0.5% to 1.50% of the chord of the airfoil and is positioned at a downward angle to the chord of between 5.degree. and 25.degree.. Preferably, the length of the flap is approximately 1% of the chord and the preferred angle to the chord is 20.degree..
In another patent, U.S. Pat. No. 4,858,852 to Preston A. Henne et al., issued on Aug. 22, 1989, and assigned to the McDonnell Douglas Corporation, it is disclosed that a special contour near the trailing edge of an airfoil improves the airfoil effectiveness. This special contour consists of a blunt base, a region of high local concave curvature, typically on the lower or higher pressure surface at or near the trailing edge, and upper surface and lower surface slopes at the airfoil trailing edge that diverge from each other. It is maintained that this trailing edge design alters the normal matched upper surface and lower surface pressures, or the Kutta condition, for airfoil section lift determination. Here it is maintained that all trailing edge devices produce surface discontinuities which produce earlier boundary layer separation, drag penalties and the loss in camber effectiveness resulting from these discontinuities.
U.S. Pat. No. 4,387,868 to Robert J. Englar et al., dated Jun. 14, 1983, assigned to the United States of America as represented by the Secretary of the Navy, entitled "Mono-Element Combined Supercritical High Lift Airfoil", discloses a multi-purpose mono-element airfoil for aerodynamic and hydrodynamic vehicles and devices. In aerodynamic applications, the airfoil is a no-moving parts high lift and cruise airfoil which in conjunction with a plenum, upon pressure initiation, causes pressurized air to issue from a slot tangent to the airfoil surface and remain attached to the airfoils shaped trailing edge, providing a controlled resultant force of thrust.
U.S. Pat. No. 3,399,848 to H. Weiss, entitled "Device For Preventing Trailing-Edge Fluttering Of Paraglider Airfoils", issued on Sep. 3, 1968 and assigned to the Doznier System GmbH describes a device for preventing luffing the undesirable large scale disorganized fluttering of flexible trailing edges of paraglider airfoils. The device consists of an aerodynamic-disturbing strip mounted on the pressure side of the wing parallel to the trailing edge thereof and at a right angle to the flexible surface. As a result of the construction, fluttering of the fabric covering in the rear area of paraglider airfoils and similar aerodynamically-active surfaces including flexible covering elements is effectively prevented within the normal range of angle of attack.
The last-named patent teaches away from the concepts known as unsteady aerodynamics, where vibrational or oscillating members can either produce destructive effects, with diminished stability and control, or produce positive additions to a vehicle's flight. The instant invention shows that, within selected parameters, unsteady aerodynamics improves the lifting characteristics of an airfoil without any proportional increase in drag.